1. Field of the Invention
The invention relates to waveform converters, with some embodiments suitable for use as waveform converters for optical communication systems.
2. Description of the Related Art
Optical fibers are widely used for communicating information, such as in large telecommunication systems, primarily owing to their reliability, their insensitivity to electrical interference, and their high data capacity. It is desirable to use fiber optic communication networks as efficiently as possible, especially when the networks are implemented over long distances. In order to transmit optical signals over these long distances, the signals generally must be amplified before transmission to compensate for transmission losses. An erbium-doped fiber amplifier, for example, is capable of directly amplifying signal light to obtain a high-intensity optical signal in the 1550 nm wavelength band, thereby enabling compensation for transmission loss in optical fibers forming optical paths, and hence, unrepeated transmission over several thousand kilometers. To increase the efficiency of transmission, the amount of information that is sent in a specific amount of time can be increased by making optical pulses as short as possible. Short pulses are advantageous in high data rate transmission techniques such as wavelength division multiplexing (WDM) and time division multiplexing (TDM).
For effective high data rate transmissions, optical signals generally require a narrow pulse width in a selected wavelength band. In some systems, dispersive and nonlinear effects in the transmission fibers can be used advantageously to modify the pulse width/shape of an optical signal. As additional signals are added to a communication path at different wavelengths, a number of signals may need to change their wavelengths in addition to modifying and/or controlling the shape of the optical pulse.
One approach to converting the wavelength of an optical signal is opto-electro-optical, wherein an optical signal is converted into an electrical signal using a photoelectric converter (i.e. photo-detector, photodiode), and the electrical signal drives a light source at a different wavelength. This method, however, entails problems such as high operating costs and difficulty in controlling the pulse width of the signal.
Additional methods for directly converting the wavelength of an optical signal utilize a semiconductor amplifier and nonlinearity properties of optical fibers. The method comprises directing the input signal at a first wavelength onto a semiconductor device, which is amplifying a steady signal at a second wavelength than the input signal. The input signal changes the amount of amplification at the second wavelength, thereby modulating the steady signal to reproduce the input signal at a different wavelength. A disadvantage, however, to using a semiconductor amplifier is a rather low signal to noise ratio.
The nonlinearity of an optical transmission medium can also be utilized such that idler light generated on the basis of a four-wave mixing (FWM) phenomenon is obtained as a wavelength converted signal. See, for example, “Interband Wavelength Conversion of 320 Gb/s WDM Signal Using a Polarization-Insensitive Fiber Four-Wave Mixer” by Watanabe, Takeda, and Chikawa, ECOC'98, September 1998, page 85. Four-wave-mixing methods, however, typically require a separate excitation light source having a different wavelength than the input signal. In addition, in order to obtain an optical signal of a desired converted wavelength, the light source wavelength must be adjusted to satisfy a phase matching condition for FWM.
Thus, it is considerably difficult to simultaneously control and manipulate both the pulse width and/or shape and the wavelength range of an optical signal effectively. It will therefore be appreciated that a device which effectively performs such a function is needed in the art.